Water-absorbing or superabsorbent polymers (SAPs, called superabsorbents for short) refer to crosslinked hydrophilic polymers that can absorb several times their mass in the dry state (sometimes more than one thousand times) of water.
The main field of use of superabsorbents is in the hygiene sector and also plays a major role in the medical sector in wound dressings and plasters. Further important fields of use for superabsorbents are agriculture and horticulture, where superabsorbents are used in order to improve the ability of soil to store moisture.
The demands on a superabsorbent depend on the particular field of use, and for that reason the properties of the superabsorbents (for example the degree of swelling and the swelling rate) have to be adjusted correspondingly. A matter of significance for this purpose is whether the absorption of the liquid to be absorbed is to take place under pressure and/or at relatively high temperature, which is especially important for the use of superabsorbents in incontinence products. Other matters of major significance are the nature and composition of the liquid to be absorbed, since the degree of swelling of a superabsorbent is significantly affected by the salt content of the swelling agent.
The water-absorbing polymers are especially polymers formed from (co)polymerized hydrophilic monomers, graft copolymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose or starch ethers, crosslinked carboxymethylcellulose, partly crosslinked polyalkylene oxide, or natural products swellable in aqueous liquids, for example guar derivatives. Water-absorbing polymers of this kind are used to produce diapers, tampons and sanitary napkins, but also as water-retaining agents in market gardening.
The production of the water-absorbing polymers is described, for example, in the monograph “Modern Superabsorbent Polymer Technology”, by F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998 or in Ullmanns “Encyclopedia of Industrial Chemistry”, 6th edition, volume 35, pages 73 to 103.
A superabsorbent polymer in the aqueous polymer gel state is regarded as being in a wet state and hence can also be referred to in general terms as wet material; in other words, the aqueous polymer gel still has a considerable proportion of water before drying; especially as described below. The aqueous polymer gel is obtained by polymerizing a monomer solution or suspension. The aqueous polymer gel with still-aqueous polymer particles is preferably introduced into the belt drier in granular form, for example with a solids content of 40-60%. In this state, the aqueous polymer gel is basically already in crosslinked form with a desired degree of crosslinking, especially in homogeneously crosslinked form at first, especially with a comparatively low degree of crosslinking, especially, as described further down, barely surface crosslinked at all at first.
A superabsorbent polymer in the a water-absorbing polymer particle state is considered to be in a state after drying; in other words, it has a low residual water content of the polymer particles after the drying of aqueous polymer gel, especially as described below; the superabsorbent polymer is thus preferably in the form of a dried polymer gel, especially dried polymer particles. In this state, the water-absorbing polymer particles can preferably be postcrosslinked, especially surface crosslinked, in which case the degree of surface crosslinking is preferably above the abovementioned comparatively low degree of initially homogeneous crosslinking. Preferably, after the polymerization, an aqueous polymer gel of the water-absorbing polymers is obtained, which is dried. The principles of drying of the aqueous polymer gel to give a water-absorbing polymer, especially dried polymer gel, comprising water-absorbing, especially dried, polymer particles is likewise described in the monograph “Modern Superabsorbent Polymer Technology”, by F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, on pages 87 to 93.
In the belt drier, the aqueous polymer gel is dried to give a partly dried polymer gel and hence takes the form of a dry cake. The dry cake preferably takes the form of a strand of partly dried polymer gel, i.e. of a partly dried polymer strand, on the belt of the belt drier which thus extends through the drier setup of the belt drier.
The dry cake, at the end of the belt drier, i.e. on leaving the drier setup, is in the form of a substantially dried strand of dried polymer gel, for instance in the form of a slab or of a sheetlike strand, i.e. of a dried polymer strand. The partly dried polymer gel and the dried polymer gel of the dry cake are sometimes already referred to hereinafter by the terminology “dried polymer particles”; both cases are covered by the terms “superabsorbent or water-absorbing polymer gel” or “dried polymer gel”, as opposed to “aqueous polymer gel”.
Given a comparatively broad size distribution of particles of the aqueous polymer gel to be dried, complete drying of all polymer particles can often be effected under drying conditions under which a majority of the particles have if anything been overdried. Ultimately, the drying process should also be economically viable and afford the desired polymer gel quality. After an appropriate dwell time of the aqueous polymer gel to be dried in the belt drier, it is to be dried to give a superabsorbent polymer comprising water-absorbing polymer particles having a desired water content, preferably low water content and hence residual moisture content. Accordingly, in an improved practice, drying conditions are preferably chosen that constitute a compromise between exploitation of the drier capacity and the processibility of the water-absorbing polymer particles.
The dried polymer gel in the form of a substantially dried polymer strand is then fed to a crusher or similar comminutor at the end of the belt drier. What are then formed are thus well-dried polymer particles of dried polymer gel.
Some of the dried polymer particles in that case take the form of crushed dried polymer gel, for example comparatively coarse lumps, and some take the form of unavoidable crush residue of dried polymer gel. In particular, the crush residue of dried polymer gel comprises fine polymer particle powder comprising fine and ultrafine particles.
The dried polymer particles are then preferably sent to a grinding operation and processed further to give ground dried polymer particles.
The ground dried polymer particles can then be sent to a sieving operation. A midsize fraction then has an already preferred desired particle size and can be separated off at this early stage. An oversize fraction or fines fraction can optionally be ground, sieved or processed once again and added to the midsize fraction.
The dried, ground and sieved polymer particles of the midsize fraction can be surface reprocessed.
The dried, ground and sieved and surface reprocessed polymer particles can be subjected to safeguard sieving.
The process preferred for the present concept is a conveyor belt process (with a conveyor belt of the belt drier). The belt drier is a convective drying system for the treatment of polymer gels through which air can flow. The aqueous polymer gel to be dried is applied to an endless, gas-permeable conveyor belt, toward which there is a flow of a preferably heated gas stream, preferably air. Drying air hereinafter means any kind of drying gas, especially air or air-like gases, preferably heated drying gases, especially heated air or heated air-like gases. In the drying operation, it is possible to use continuous convection belt driers; this relates hereinafter to a belt drier of the type specified at the outset, especially an air circulation belt drier. The belt drier mentioned at the outset is designed particularly for drying of a deformable, pasty product of limited flowability and in piece form, especially for an aqueous polymer gel. In a continuous belt drier, the polymer gel layer of an aqueous polymer gel, applied in the form of an aggregate through which air can flow, on a perforated conveyor belt is transported through the drying space and dried in the process at first to give partly dried polymer gel and finally to give dried polymer gel; the latter is then processed further to give the abovementioned dried polymer particles as water-absorbing polymer particles.
The drying gas that flows through the product layer of the dry cake of partly dried polymer gel and then dried polymer gel serves both to introduce heat into the aqueous polymer gel to be dried or into the partly dried water absorbing polymer particles and to transport away evaporating moisture; in principle, this is achieved in that a temperature in the drier increases at least in a subsection in line with the conveying direction of the polymer gel, and the moisture content of the aqueous polymer gel to be dried decreases in conveying direction of the conveyor belt. The drying gas used is preferably air as drying air. In the case of a belt drier cited at the outset, in that case especially in the form of an air circulation belt drier, the drying air that flows through the polymer gel layer can also be conducted as circulating air; this can especially also be supplemented with fresh air and thus replaced fully or partly as required; in principle, supply of pure fresh air, however, seems to be less energy-efficient than intelligent air recycling; especially from an end zone from which the recycled air has comparatively low moisture loading in upstream zones of the drier.
Compared to other designs of drier, the belt drier has the advantage that (apart from gravity) there is no significant mechanical stress that impairs the polymer gel, since the aqueous polymer gel or the water-absorbing polymer particles lie loose on a conveyor belt. In principle, it is possible to configure the construction of a belt drier with a single drier zone. In one modification, it is also possible to configure the construction of a number of drier zones in full or in part. In the simplest case, the drier setup comprises a single drier zone; or in a more complex case a number of drier zones.
A drier zone may, but need not, have a modular construction, i.e. be constructed by means of a single drier module or a multitude of drier modules. Ultimately, a drier setup can be constructed by means of a number of drier modules. A belt drier comprises, for example, a polymer gel application module, a number of drier modules for formation of one or more drier zones, and a discharge module.
The discharge module serves to discharge the superabsorbent polymer in the form of the water absorbing polymer particles; more particularly, the conveyor belt ends, or has a turning point, in the discharge module; the superabsorbent polymer in the discharge module may fall onto an abovementioned crusher or similar comminutor.
Belt driers having transport belts are to be distinguished from belt reactors. While a belt reactor is used to produce aqueous polymer gel from its starting materials, a belt drier is used to produce water-absorbing polymer particles from an aqueous polymer gel, especially to produce the water-absorbing polymer particles mentioned, preferably from an aqueous polymer gel that has first been homogeneously crosslinked with the desired degree of crosslinking, and optionally also surface crosslinked.
WO 2006/100300 A1 discloses a process for preparing water-absorbing polymers by polymerization of a monomer solution and drying of the aqueous polymer gel obtained by the polymerization in a belt drier by means of a heated air stream, wherein the drying is conducted in at least two temperature zones and/or the flow direction of the drying air stream through the aqueous polymer gel is conducted from beneath in the upstream section of a belt drier and from above in the downstream section of the belt drier. This involves circulating the drying air for it to undergo maximum saturation in multiple passes through the polymer gel layer. For economic drying of the water-absorbing polymers, the air flow regime in the drier is systematically designed for energy-efficient operation. Various air flow regime concepts that have advantages in terms of drying characteristics and energy exploitation are possible: in crosscurrent from the top downward, alternating, cross-countercurrent or else in cross-cocurrent. Preference is given here to an overriding air flow regime in cross-countercurrent. The drying is preferably conducted at a pressure reduced relative to atmospheric pressure. This reduced pressure, as the differential from atmospheric pressure, is preferably at least −0.5 mbar, more preferably at least −2 mbar, most preferably at least −10 mbar (minus sign indicates reduced pressure). The reduced pressure in the drier relative to atmospheric pressure brings about more favorable gas flow in the drier and hence more homogeneous drying. WO 2008/034786 A1 describes a process for producing color-stable water-absorbing polymer particles having a lower degree of neutralization, in which a process is conducted by drying in at least two temperature zones.